Morphology and mobility of oyster hemocytes: evidence for seasonal variations.

Hemocytes of Crassostrea virginica were video recorded and tracked to determine their locomotive rates and to assign these rates to Wright-stained morphological variants. From 24 oysters examined in January, February, March, and May, 1571 hemocytes were video recorded, identified, and their rate of locomotion (ROL) measured. Granulocytes (three types) and agranulocytes (one lymphoid and three nonlymphoid types) were recognized. Focusing on 15 oysters in March and May, 20,318 hemocytes were counted from duplicate slides to verify the classification and to show that predominant hemocytes vary greatly between samples and among individual oysters, yet population differences can be detected. Measured rates of locomotion indicate that the granulocyte subpopulation moved significantly faster (3.3 microns/min) than the agranulocyte subpopulation (0.7 microns/min) because most (81%) agranulocytes were not mobile. Of the mobile hemocytes, granulocytes were also significantly faster (4.8 microns/min vs. 3.5 microns/min, P less than 0.0001), and basophilic granulocytes (BASOs) were the most active and abundant cell type. Examination of monthly percentages of cells and ROL indicates, however, that granulocyte dominance and ROL are not invariable. Granulocyte percentages of more than 60% in January, February, and March decreased to 32% in May, and BASO dominance was reduced to 15%. Further, percentages of mobile granulocytes decreased from greater than 65% in January, February, and March to 50% in May. ROL for all cells decreased from greater than 2.3 microns/min in these months to 1.0 microns/min in May. The fewer mobile hemocytes tracked in May had significantly (P less than .05) lower average ROL (4.0 microns/min) than those in January and March (4.7 microns/min each). Agranulocytes increased in May due to an increase in nonlymphoid cells.